Growth hormone (GH) promotes the tyrosyl phosphorylation of its receptor and other cellular proteins. Tyrosine kinase activity also co-purifies with the GH receptor. These results suggest that activation by GH of a GH receptor-associated tyrosine kinase may be an important step in the signal transduction mechanism for GH. Our research is now focused upon cloning the GH receptor-associated tyrosine kinase and using in vitro mutagenesis to determine its role in celluar actions of GH.